INFRARED AND THERMAL CONDUCTIVITY CO SENSORS IN …

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1) TemperatureNormal temperature for the human body, 37 degrees Celsius, is

an optimum temperature for most cell cultures, although some

might require different temperatures. Cells must stay within a

narrow temperature range — a few tenths of a degree — to avoid

conditions that threaten the cell culture or create a significant

delay in growth and impact on schedules.

2) Relative HumidityRelative humidity (RH) levels in the incubator prevent growth

medium desiccation. RH can be as low as 75 to 80 percent.

More commonly, RH must remain above 90 percent.

3) Carbon Dioxide (CO2)Different cells grow best in environments with pH values that

typically range from 7.0 to 7.7. Growth medium includes a

pH buffer, often CO2-bicarbonate-based, to keep conditions

stable. Chemical reactions in the medium can alter the pH.

Control of atmospheric CO2 levels helps maintain steady

growth-medium pH.

Measuring CO2There are two main technologies for measuring

CO2 — infrared (IR) and thermal conductivity. This paper

compares them and examines which technology might

be best for a given laboratory.

Incubator StructureAn incubator is essentially a box within a box. The out-

ermost shell [A] is what remains visible when the door

is closed and the system operating. Within that shell is

the growth chamber [B], in which temperature, relative

humidity, and CO2 are controlled.

All three conditions require control because they affect cell

growth. Heating elements, whether directly applied to the

growth chamber or indirectly through a water-filled jacket

surrounding the chamber, maintain a temperature, typically

37 degrees Celsius, although, depending on the specific need,

temperatures can range significantly higher or lower. A water

reservoir or pan provides humidity. Both temperature and

humidity are of secondary importance in this discussion.

An incubator also needs control of CO2 content in the air,

generally achieved through an exogenous source of CO2, like

an external tank [C] of gas attached through a hose to the

incubator. A sensor, using either IR or TC technology, monitors

the level of CO2, activating a mechanism that adds more of the

gas when levels are too low.

HOW TO CHOOSE BETWEEN INFRARED AND THERMAL CONDUCTIVITY

CO2 SENSORS IN INCUBATORS

Incubators, also called carbon dioxide (CO2) incubators, are key equipment for biological and medical laboratories. They enable the necessary environmental control and isolate cell cultures from external conditions and contamination. Control focuses on three major factors:

[A]

[C]

[B]



Chemistry of pH and Cell GrowthThe reason CO2 control is critical in incubators is that it offers

indirect control of pH levels.

Cells are grown in a container, whether a petri dish, a 96-well

plate, or some other type. A liquid, solid, or semi-solid growth

medium provides nutrients and a substance the cells can

fasten to. Growth media typically include a pH buffer, often

carbonate-based, to keep pH levels stable as cell processes

throw off acids as byproducts.

As a reminder, pH, which stands for potential of hydrogen, is a

measure of acidity on a scale from 0 to 14. Specifically, it is a

measure of the number of hydrogen ions, symbolized by H+, in

a solution. Below is the formula for pH:

pH = -log10[H+]

Cells that researchers study and cultivate tend to thrive in a

relatively narrow pH range of about 7 (the “normal” level of

pH found in unaltered water) to 7.7 (slightly alkaline). If the pH

ranges too far above or below the optimum level for a type of

cell, at best growth will slow and affect lab work and schedules.

If the pH level is off by too extreme an amount, the culture

will die.

Role of CO2 in Incubator pH Although growth media frequently include pH buffers,

commonly used carbonate-based buffers depend on a balance

between CO2 and bicarbonate dissolved in the medium. If CO2

escapes into the atmosphere, alkalinity increases and the pH

level changes.

The most common method to prevent release of CO2

into the atmosphere of the growth chamber is to ensure a

sufficiently high percentage of the gas in the air. This is

due to the physics of gases, including the concept of partial

pressures, each separate gas in a mixture exhibiting a portion

of the overall pressure of the mixture, and a principle called

Henry’s Law. It states that the concentration of a gas that is

dissolved in a liquid, like the CO2 in the growth medium [D], is

directly proportionate to the partial pressure of that same gas

in the atmosphere above the liquid [E].

In other words, for a particular medium and type of buffer,

a sufficient partial pressure of CO2 in the growth chamber

atmosphere prevents more CO2 from escaping the medium. As

a result, the pH of the medium stays controlled.

In practical terms, given normal atmospheric pressure, the

incubator only needs to keep the CO2 level sufficiently high as a

percentage of the overall atmosphere. A commonly used value

is 5 percent, compared with the normal roughly 0.3 percent of

CO2 found in the atmosphere at sea level. The exact percentage

needed will vary with the type of growth medium.

If too little CO2 is in the atmosphere, CO2 will be able to escape

from the growth medium and the mixture will become too

alkaline. Too much CO2 in the atmosphere will enable more of

the gas to be absorbed by the medium, ultimately turning it

too acidic.

No matter what the necessary value, the incubator must

be able to calculate the amount of CO2 in the atmosphere

accurately and quickly.

How to Choose Between Infrared and Thermal Conductivity CO2 Sensors

[D]

[E]

Henry’s Law states that the concentration of a gas that is dissolved in a liquid is

directly proportionate to the partial pressure of that same gas in the atmosphere

above the liquid.



CO2 Sensor TechnologiesThere are two technologies in use that measure the percentage

of CO2 in the atmosphere of the growth chamber.

Thermal Conductivity (TC)TC sensors work through measuring electrical resistance

through the air. Typically, a sensor is composed of two cells,

each containing a thermistor, a “thermal resistor” in which the

resistance changes with atmospheric conditions, including gas

composition, temperature, and humidity. A sealed reference cell

[F] encloses reference atmosphere at a controlled temperature

while the other cell [G] can be filled with the growth chamber’s

atmosphere.

Circuitry measures the difference in resistance between the

two cells. When the chamber atmosphere is in a steady physical

state and the system is calibrated to known temperature and

humidity conditions, the difference in resistance between the

reference cell and the other is the result of the difference in

CO2 concentration.

TC sensors offer a substantial price savings by as much as 20

percent of the device cost. However, they have a significant

drawback. Because thermistor resistance varies with tempera-

ture and relative humidity, as well as gas composition, any time

the door to the growth chamber is opened, the temperature

and humidity change from what the calibrated sensor expects,

which means the readings are no longer accurate.

Once the door is closed again, regaining a steady state of

temperature and relative humidity can take as long as 40

minutes for the typical incubator. Until then, any reading is

unreliable. Some vendors begin to add CO2 after a door opens

to regain levels, but because they don’t know what the existing

levels are, the percentage may be incorrect, affecting the pH

balance of the medium. In some cases, like medical and pathol-

ogy uses, this may not matter. But for many types of research,

the inaccuracy can lead to poor reliability and consistency in

measurement and in results.

Infrared (IR)IR sensors rely on the fact that each gas absorbs a distinct

wavelength of light. CO2 absorbs the wavelength 4.3μm, within

the infrared portion of the Electromagnetic spectrum.

An IR emitter [H] directs infrared light [I] through a sample of

the growth chamber’s atmosphere, then through a Fabry-Perot

Interferometer [J] filter which isolates the proper wavelength,

and finally into a sensor [K]. Periodically calibrated circuitry

measures the amount of 4.3μm light that strikes the sensor and

calculates the difference between it and what was emitted by

the source.


SAMPLEINFLOW

SAMPLEOUTFLOW

[F] [G]

Thermal Conductivity Sensors measure the difference in electrical resistance

between a sealed reference cell and a cell open to the chamber atmosphere. C02

content is calculated based on the difference in resistance between the two cells.

MirrorSurface

ProtectiveWindow

[H]

[I]

[J][K]

Infrared (IR) C02 Sensor



The more CO2, the less light passes. The difference allows the

circuitry to calculate the percentage of CO2.

Because the light absorption does not depend on temperature

or humidity, the sensor is accurate any time, including shortly

after opening and closing the door of the growth chamber.

Making the ChoiceAn incubator with a TC sensor is financially tempting to any lab,

as the price difference can be substantial although in recent

years the cost of IR sensors have decreased making them more

affordable. If the lab’s work is of a type that will feel little effect

from CO2 reading inconsistency and inaccuracy every time the

incubator door is open, the choice might make sense.

However, for labs that either do more sensitive work now or

might in the future, an IR sensor-equipped incubator is the right

choice. Aside from the loss in accuracy in work product with

TC, which could affect quality, there is a subtle but substantive

financial argument for IR.

If personnel cannot trust the readings of the CO2 sensor for

even half an hour after a door is opened, they lose productive

time. You can calculate the effective cost to the organization.

Multiply the typical number of times the door is opened a day

by a half-hour. Determine the fully loaded cost of a research-

er’s time. Add the expected margin for an employee’s time. The

total is the total cost to the organization. For example, say the

door is opened four times a day and a researcher’s time is ef-

fectively worth $150 an hour. The resulting two hours a day is

worth $300, or $1,500 of lost productivity a week. That would

be $6,450 for the average 4.3-week month and $77,400 a year.

The money “saved” by using a TC sensor-equipped incubator

quickly evaporates. Within just a month or two, the total cost of

the IR sensor-equipped model is lower.

For most labs, especially ones looking to expand either their

type or amount, of work IR sensors make sense, both financially

and qualitatively.

NuAire Laboratory Equipment SupplyNuAire manufactures ergonomically designed and engineered

scientific laboratory equipment providing personnel,

product and/or environmental protection in critical research

environments. NuAire’s extensive line of laboratory equipment

includes:


To learn more or to speak with someone at NuAire please visit nuaire.com or call 763-553-1270.

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